1. Field of the Invention
The present invention relates to a refrigeration lubricant composition that has a polyol ester as a main component and has excellent long-term low-temperature stability and lubricity, and a refrigerant working fluid consisting of the composition and a chlorine-free hydrofluorocarbon refrigerant, and a refrigerating apparatus employing the composition.
2. Description of the Related Art
Conventionally, refrigerants containing chlorofluorocarbon have been used for air conditioning equipment such as room air conditioners and packaged air conditioners, low temperature apparatuses such as refrigerator-freezers for home use, industrial refrigerators, and automotive air conditioners such as hybrid cars and electric cars. However, due to problems such as depletion of the ozone layer, the replacement of such chlorofluorocarbon refrigerants with chlorine-free hydrofluorocarbon refrigerants such as those containing 1,1,1,2-tetrafluoroethane (R-134a), pentafluoroethane (R-125), difluoroethane (R-32), and mixtures thereof has been promoted. Consequently, a variety of refrigeration lubricants containing a polyol ester, which has good compatibility with chlorine-free hydrofluorocarbon refrigerants, as a base stock have been proposed.
In view of hydrolytic stability and compatibility with chlorine-free hydrofluorocarbon refrigerants, hindered esters having excellent thermal resistance have been put to practical use for refrigeration lubricants. These hindered esters are derived from a carboxylic acid having a methyl branched chain or ethyl branched chain in the α position or β position and pentaerythritol. For example, Japanese Laid-Open Patent Publication No. 10-8084 discloses refrigeration lubricants having an ester comprising pentaerythritol and a mixed monocarboxylic acid of 2-ethylhexanoic acid and 3,5,5-trimethylhexanoic acid as a main component and discloses that such refrigerants have good stability at high temperatures.
In recent years, energy saving is required also from the viewpoint of environmental problems, and therefore a demand for improving lubricity has become greater and greater than ever. Although esters derived from straight-chain monocarboxylic acids are known as esters that provide a high lubricating effect, such esters have disadvantages in that they tend to be crystallized at low temperatures.
Japanese Laid-Open Patent Publication No. 11-228984 discloses refrigeration lubricants that comprises an ester that can be obtained using raw materials made of a combination of straight-chain monocarboxylic acid and branched chain monocarboxylic acid have excellent lubricity. However, this lubricant is not sufficient to solve the problem that crystals occur at low temperatures, either.
In the refrigeration cycle, a part of the refrigeration lubricant generally circulates through the cycle along with a refrigerant, and thus the refrigeration lubricant is exposed to a high temperature region and a low temperature region. When the refrigeration lubricant is crystallized in a low temperature region, the amount of the circulating refrigerant in the refrigeration cycle is reduced, which may lead to problems such as poor cooling. Therefore, such high stability that prevents crystals from being precipitated over a long time at low temperatures is required for refrigeration lubricants. That is, to develop polyol ester as a refrigeration lubricant, there is a demand for long-term stability at low temperature along with improvement of lubricity, and it is very important to attain these two contradicting performances at high levels.
Thus, there is a demand for development of refrigeration lubricants having good lubricity and having high stability that prevents the refrigeration lubricant from being precipitated for a long term at low temperatures.